Generally, a concave grating monochrometer has heretofore been used to disperse the radiation light having wavelengths of from about 5 angstroms to about 2000 angstroms over the range of soft X-rays through up to vacuum ultraviolet regions, and to take out a given wavelength component. This is because, in these wavelength regions, the light is reflected poorly by the surface of metal and, besides, a suitable lens material is not available. Therefore, it is difficult to use the plane grating which necessitates an auxiliary optical system for collimation or for focusing the light. The concave grating having both the dispersion power of the plane grating and the focusing power of the concave mirror, therefore, is effective for dispersing the light over these wavelength regions.
However, in the conventional concave grating that had hitherto been used from the 1880s up to the present time, constant spacing and straight grooves are engraved in the concave spherical surface. Therefore, due to the limitation in the arrangement of grooves in the grating, the spectral image obtained by the spectrometer inevitably contains much aberration. With the concave grating monochrometer, therefore, there is obtained a single wavelength component having a low wavelength purity, and the spectral efficiency is low.
In order to solve such problems, a system has recently been proposed according to which the grooves of the concave grating are arranged in a varied spacing and curved manner, and the distance among the grooves of the grating and the curvature of grooves are selected depending upon the wavelength regions of the spectrometer or the arrangement of the optical system, such that the aberration, that was not avoidable with the conventional concave grating, can be removed or greatly reduced (see M. C. Hutley, Diffraction Gratings, Academic Press, 1982, p. 232; Japanese Patent Publication No. 33562/1982).
Such a so-called aberration corrected concave grating can be prepared by either the mechanical ruling or by the so-called holographic method in which interference fringes by a laser beam are photographically processed, to arrange the grooves in the grating maintaining a very high precision. In this case, however, if there exists error in the shape of a concave spherical surface that serves as a grating substrate, there develops aberration in the spectral image to a degree equivalent to the error in the grating groove arrangement.
Generally, it is difficult to prepare a spherical surface that works as a concave grating substrate compared with a plane. In forming a spherical surface having a desired curvature, in particular, error is frequently involved in the radius of curvature and in the spherical surface. Therefore, despite the grooves are arranged in the grating in the same manner as the plane grating, the concave grating is not capable of exhibiting spectral imaging function to a degree which is comparable to that of the plane grating.